1. Field of the Invention
The present invention relates to a method of forming a thin plate and its forming die.
2. Description of Related Art
Negative angle forming of a thin plate such as a sheet metal or a plastic plate is performed using a slide cam. In the meantime, the aforementioned "negative angle forming" means such forming which produces a formed portion that enters a lower die from a working locus of an upper die when a work placed on the lower die is formed by pressing with the upper die vertically going down. Conventional negative angle forming processing for thin plate products was performed in such a way that a work is placed on the lower die, and the upper die is made to go down vertically to drive a lower die receiving cam by an upper die actuation cam, and the work is processed from the direction of its side. When processing finishes and the upper die rises, the receiving cam is withdrawn by a spring. In this case, the forming portion of the receiving cam which slides in from a position apart from a transversal outside of the work and forms the work is formed into the same and integrated shape as that of the forming portion of the work. However, the forming portion of the lower die to be loaded with the work must be taken out from the lower die after processing the work, thus the negative angle portion of the lower die is divided to withdraw or the back of the negative angle portion is cut and the work must be moved forward to enable the work to be taken out. When a negative angle is small, there is almost no problem. However, when a negative angle is large, or the product has a long and narrow frame shape with a section of a slot, for example, a front pillar outer of an automobile's sheet metal parts, if the negative angle portion of the lower die is divided or cut, a clear shape not only can not be formed on the forming portion of the receiving cam because of the narrow slot width of the work, but also the negative angle forming processing was impossible because of insufficient strength of the lower die. Moreover, the negative angle forming by a slide cam is performed by sliding a receiving cam over a considerably long straight distance, thus it is not necessarily easy to repetitively slide the receiving cam to a predetermined position, therefore it is difficult to obtain products of stabilized quality. Still furthermore, torsion or deformation arises in the products and sometimes it is required to repair the products, however parts constituting an automobile's shell plate portion such as a side panel, front fender, roof, hood, trunk lid door panel and front pillar outer, all of them have three-dimensional curved surfaces and curves, and thus repairing these products is practically impossible. In the case of an automobile's sheet metal assembly, if there is torsion or deformation in parts, these parts are difficult to connect with other parts, and thus an automobile's sheet metal structure of high quality could not be supplied, therefore it was impossible to maintain predetermined accuracy of thin plate forming parts. Furthermore, use of the slide cam necessitated equipping a large receiving cam or a heel on a side of the lower die load with a work, thereby increasing the area of the lower die, that is, the weight of the die, and also resulting in an expensive die.
Then, in order to solve the above problems, the present applicant has already proposed, in patent publication SHO 63 (1988)-41662, a method of forming a thin plate and its forming die in which vertical linear motion of a press is converted into revolution.
First, the forming die is explained with reference to FIGS. 6-10.
FIG. 6 is a schematic perspective view showing finished right and left parts of front pillar outer of automobile sheet metal parts formed by the present forming die. In FIG. 6, a part shown below is for right side use and a part shown above is for left side use. The front pillar outer is a part of a front door frame and also a part of a front glass frame and furthermore a part of a roof panel supporting frame. Therefore, it has a number of connections with many parts and a product which is required to have severe accuracy and when its accuracy is not attained, a sheet metal body of good quality can not be obtained. In addition, this front pillar outer has three-dimensional curved surfaces and curves. Portions to be negative angle formed by the present forming die are shown with characters F in FIG. 6, and the section of the portion is shown as a work W in FIG. 10. That is, a state of the work W shown in FIG. 7 becomes a state of the work W shown in FIG. 10. In this connection, in processes of this press working, first drawing is performed, next outer peripheral trimming is effected to the extent shown in FIG. 7, and then the present forming process is carried out as the third process.
A lower die 100 is provided with, rotatably in a lower die body 103, a cylindrical rotary cam 102 having a slot 101 engraved in the direction of its axis. The lower die body 103 is fixed on a lower base 121 with a bolt 122. The top face of the lower die body 103 is formed into a shape capable of placing a work W, and a negative angle forming portion 104 is formed on the slot 101 edge portion of the rotary cam 102 near the top face of the lower die body 103. An automatic return device 105 which rotates and withdraws the rotary cam 102 so as to take out the work W from the lower die body 103 after forming it is buried in the lower die body 103. In this embodiment, the automatic return device 105 makes a push pin 107, which is activated by a coil spring 106, contact an end bottom face of a rotary plate 108 which is fixed with a bolt 151 to a face opposed to the negative angle forming portion 104 of the slot 101 of the rotary cam 102. In addition, as an automatic return device 105, a pneumatic system, hydraulic system, link mechanism, cam or the like can be used, and the device can be provided between the upper and the lower dies 109 and 100 as well as in the lower die 100.
The upper die 109 is provided with a slide cam 110 in a position opposed to the rotary cam 102. This slide cam 110 is provided with a negative angle forming portion 112 at its bottom end. The slide cam 110 is guided by a guide (not shown) and activated toward an outside of the die by a coil spring 117 compressed between the top face of the slide cam 110 and a bottom face of an inclined guide 154 fixed to an upper base 152 with a bolt 153. The slide cam 110 is stopped by a stop plate 156 fixed with a bolt 155 to the inclined guide 154. A pad 157 is activated downward by a coil spring 158 and is hanged from the upper base 152 by a hanging bolt 110 and pushes the work W strongly against the lower die body 103 so as not to move the work W before negative angle forming of the work W.
Next, the performance of this forming die will be described.
As shown in FIG. 7, the upper die 109 is first positioned at a top dead center, and at that time the work W is placed on the lower die body 103 of the lower die 100. At this time, the rotary cam 102 has been rotated and withdrawn by the automatic return device 105.
Then, the upper die 109 starts to go down, and as shown in FIG. 8, first the bottom face of the slide cam 110 contacts the rotary plate 108 without the slide cam 110 interfering with the negative angle forming portion 104 of the rotary cam 102 and rotates the rotary cam 102 clockwise in FIG. 8.
When the upper die 109 still continues to go down, the slide cam 110 which is activated toward an outside of the die is moved to the left in the transverse direction by the action of the cam against the activating force of a coil spring 117 and reaches a state shown in FIG. 9, and thus the work W is negative angle-formed by the negative angle forming portion 104 of the rotated rotary cam 102 and the negative angle forming portion 112 of the slide cam 110.
After the negative angle forming, the upper die 109 starts to rise. The slide cam 110 has been activated toward an outside of the die by the coil spring 117 and moves right in FIG. 10 and still rises without interfering with the negative angle-formed work W.
While the restricting slide cam 110 rises, the rotary cam 102 is rotated counterclockwise in FIG. 10 by the automatic return device 105, and the work W can be taken out without interfering with the negative angle forming portion 104 of the rotary cam 102 when taking out the negative angle-formed work W from the lower die body 103.
As mentioned above, the thin plate forming parts nave been negative angle-formed using a rotary cam, and the rotary cam processes the work, rotating on its axis, thus viewing from the direction of processing, when the work is almost straight, the negative angle forming portion is housed in one rotary cam and can be processed. On the other hand when the work is bent, the negative angle forming portion can not be wholly housed in one rotary cam, and thus can not be processed.
In particular, sheet metal parts of an automobile including a door panel have many bent portions, moreover in recent years, have many negative angle-formed portions besides the bent portions due to the automobile's design. Therefore, it is desired that the negative angle formed portion can be formed in only one process without passing it through many processes, whereby the production efficiency is improved.